fix: update keyboard nav to use new clipboard APIs

[BeksOmega]

Description

Updates keyboard nav to use the API added in RaspberryPiFoundation/blockly#5996 so that we're no longer using interal properties.

Also updates the plugin to use Blockly.clipboard.copy instead of Blockly.copy

Testing

1. Opened the keyboard nav test playground.
2. Added an if block to the workspace.
3. Enabled keyboard nav.
4. Copied the block.
5. Selected the next input of the if block.
6. Pasted.
7. Observed how the pasted block was properly attached to the bottom of the original block.

All unit tests pass.